Advancements in the field of terahertz (THz) science and technology have resulted in a number of important applications such as security and medical imaging [1], explosive detection [2], non-destructive testing [3] and wireless communication [4]. Research has been driven by the unique properties of THz radiation: it is non-ionising thus safe to biological tissue; transparent to several common plastics and fibres; and has a shorter wavelength than millimeter waves, giving a higher spatial resolution. In addition several materials, such as explosives and illicit drugs, have characteristic THz spectroscopic signatures that can be readily identified [5, 6]. The proliferation of these applications into everyday life has been hampered by the lack of inexpensive, compact and room-temperature THz sources and detectors.
Known THz detectors typically comprise discrete components that are bulky and exhibit a low level of integration at high cost. Common THz detectors include pyroelectric sensors [7], Schottky barrier diodes [8] and field-effect transistors [9]. However these detectors have one or more limitations such as low sensitivity, slow speed, the requirement for cryogenic cooling and difficulty scaling to array formats required for THz imaging. CMOS transistors are also capable of detecting THz radiation however the detection efficiency drops off markedly above 590 GHz [10]. Most terahertz images today are built up one pixel at a time, through raster scanning with single-point detectors Infrared FPAs have been used in THz imaging experiments however the detection efficiency is typically less than 5% [11].
The lack of natural frequency selective materials that absorb THz radiation has prompted researchers to explore artificial means such as the use of metamaterial (MM) devices [12]. Metamaterials are sub-wavelength elements, the structure of which rather than their composition, dominate their electromagnetic properties. Applications include cloaking [13], superlensing [14] and plasmonically induced transparency [15]. MMs can also be used to create resonant absorber structures where the radiation is absorbed in a device thickness of <λ/30, overcoming the thickness limitation of traditional quarter wavelength devices [16-18].
Optical imaging uses a range of different materials to acquire desired electromagnetic responses for unique applications. However, we are limited by properties of natural materials and must alter their shape or composition to make them suitable for a given purpose. For example, glass must be shaped to act as a lens. Modern nanofabrication techniques give us a method to further extend this concept, such that microscopic changes we make to a material can have macroscopic effects. Two such materials that utilize this method of controlling optical properties are metamaterials (MMs) [28, 29] and plasmonic devices [15].
MMs are micro/nano-structures with an engineered electromagnetic response. Geometric structure and constituent materials can be changed to alter the effective electrical permittivity ∈eff and the effective magnetic permeability μeff of the bulk material, which determines the electromagnetic behaviour of the device [28, 29]. Through manipulation of the effective bulk material parameters, MMs can be exploited to create perfect lenses [30], invisibility cloaks [31, 32] and perfect absorbers, theoretically capable of having unity absorption [33].
A large range of naturally occurring materials are capable of absorbing radiation within unique frequency bands. Filtering methods can further increase the specificity of detectors and this is the foundation of colour imaging. However, we are limited in the frequency regions of the electromagnetic spectrum that natural materials are capable of detecting. One such region where frequency selective detection is difficult is known as the “terahertz gap”, ranging from approximately 0.1 THz to 10 THz. Cleverly designed metamaterials have been shown to be particularly adept terahertz absorbers [18, 19, 21, 34] and through integration with bolometric sensors [35, 36], have potential in terahertz imaging applications.
The fundamental physical process underlying plasmonic devices is known as surface plasmon resonance (SPR), where incident light resonantly couples with surface plasmons (SPs) at a metal/dielectric interface [15, 37-39]. Optical components can be manufactured by exploiting SPR including colour filters [40, 41] and lenses [42-44] which, through design and fabrication, demonstrate a highly controllable electromagnetic response.
Momentum mismatch between the incident light and SPs must be bridged before coupling can occur. Ebbesen et al. reported on the effect of a periodic array of sub-wavelength holes on silver film [15]. Enhanced transmission of visible light through the holes and wavelength filtering was observed due to the hole array providing the additional momentum for SPR. Varying the period, size and position of the holes can alter the filter performance [37, 38].
Plasmonic filters are an excellent alternative to conventional dye filters in complementary metal oxide semiconductor (CMOS) imaging. Dye filters have been shown to exhibit cross talk between photodiodes as the CMOS pixel size scales below 2 μm due to the large distance between filter and photodiode [45]. However, plasmonic filters can be fabricated in a metal layer as part of the CMOS process, thereby decreasing cross talk and fabrication complexity [46-48].
A burgeoning branch of MMs currently provoking wide interest is the topic of so called MM perfect absorbers. By manipulating the effective electrical permittivity, ∈, and magnetic permeability, μ absorption close to unity is possible [60]. Traditional electromagnetic absorber structures such as Jaumann absorbers or Salisbury screens [61] require a thickness of λ/4, where λ is the centre wavelength of the incident electromagnetic radiation. MM absorbers, on the other hand, can absorb the incident radiation in far thinner layers, typically λ/35 [33]. The motivation for studying MM absorbers lies in their potential applications as selective thermal emitters [62], spatial light modulators [63, 64] and in detection and sensing [65, 66]. Single band, dual band and broadband MM absorbers have been demonstrated across all the major wavebands; from the microwave [33], THz [60, 19-21], IR [23] and visible [67]. An extensive overview of the topic can be found in [68].